Astron. Astrophys. 348, L41-L44 (1999)

2. Observations

Imaging data were obtained at the CTIO 1.5 m on 13 November 1998, in Johnson B and R, and Cousins I. In each band, three images were taken of 5 min exposure time each. The seeing was somewhat variable: in B, in R, and in I. Pixel size was . Small sections of these images are reproduced in Fig. 1, revealing the multiple structure of the QSO already at first glance. Two almost merging images (A1 and A2; see Fig. 2) of nearly equal brightness dominate the total magnitudes. Another discrete component, B, is clearly apparent with roughly similar colours. Components C and D are much less prominent, and in fact appear as two distinct sources rather than just one only in I.

Fig. 1. BRIK band images of the multiple QSO system. Each subimage measures ; north is up, east is to the left.

Fig. 2. Left panel: Adopted nomenclature and measured positions of the five components. Filled black circles give the positions from Table 1, open circles show individual measurements in the different photometric bands. Right panel: Contour plot of residual I band image after subtraction of five point sources. The dashed contour shows the zero level, while negative residuals are dotted, and positive residuals are surrounded by the solid contours. Contour difference is the photon shot noise in each pixel.

On the same night, near-infrared ( band, 2.0-2.3 µm) images were taken at the CTIO 4 m telescope with the CIRIM imager. Total exposure time was 18 min under seeing. The pixel size of the raw data was , but the single exposures were combined on a finer grid; the resulting image is also shown in Fig. 1. The centrally located component D is now almost as bright as A1 and A2 and unambiguously separated from C. A quantitative analysis of the astrometric and photometric properties is given in the next section.

At the time of these observations, HE 0230-2130 had been a mere QSO candidate based on a digital objective prism spectrum, with an estimated redshift of . Although we had little doubt about its QSO nature, confirmation was clearly needed. We obtained a first low-resolution spectrum on 23 November 1998 with the ESO/Danish 1.5 m telescope equipped with DFOSC. A slit of positioned East-West gave a spectral resolution of Å FWHM. The rather poor seeing of inhibited all attempts to separate different components; and the spectrum, displayed in Fig. 3, represents more or less the superposition of all components. It shows a typical QSO with , as measured from the centroid of the Mg II emission line.

Fig. 3. Spectrum of HE 0230-2130 taken with the ESO/Danish 1.5 m telescope at poor seeing; the spectrum is the sum of all components.

An attempt to obtain individual spectra of different components was made at the ESO 3.5 m NTT on 10 December 1998, using the red arm of EMMI with grism #3 and a slit, at 8 Å resolution. Only one spectrum could be taken, with the slit oriented North-South crossing A2 and B. At a seeing of , the components were well separated, showing two QSO spectra at the same redshifts. Unfortunately, the spectrum of B suffers from substantial slit losses: While the flux ratio A2/B is 2.3 in the red, in good agreement with the PSF photometry described below, it increases to at the blue end around 4000 Å. To correct for these losses, the quotient spectrum was fitted with a 5th-order polynomial, and the spectrum of B was multiplied by the fit, thereby adjusting both spectra to the same global level of relative fluxes. The result is displayed in Fig. 4 and discussed below.

Fig. 4. Comparison between the NTT spectra of components A2 and B (slightly smoothed). The thick line shows A2, the thin line shows B after correction for slit losses (see text). Below the unsmoothed difference of both spectra is plotted together with the envelope expected from pure shot noise (dashed line).

© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999
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